1. Field of the Invention
The present invention relates to a light exposure controlling method for controlling a light exposure for projecting predetermined pattern to a photosensitive substrate, for example, in exposure apparatus used in the photolithography process for producing semiconductor devices, solid state image sensing devices, liquid-crystal display devices, thin-film magnetic heads, or the like, which can be applied to a full wafer exposure apparatus, but which is suitably applicable particularly to scanning projection exposure apparatus of the step-and-scan method or the like.
2. Related Background Art
For producing the semiconductor devices etc., the projection exposure apparatus has been used heretofore for projecting a pattern of a mask on a reticle through a projection optical system to each shot area on a wafer (or a glass plate or the like) coated with a photoresist. One of fundamental functions in such projection exposure apparatus is a light exposure controlling function for keeping the light exposure (total light energy incident on the surface per unit area) at each point in each shot area of the wafer within an appropriate range.
The light exposure control in the full wafer projection exposure apparatus such as the conventional steppers was basically the cut-off control in either case where the illumination light source was a continuous light source such as an ultra-high-pressure mercury lamp or a pulsed laser light source such as an excimer laser light source. This cut-off control is such a control that during light irradiation to a photosensitive material coated on the wafer, part of the light is branched away to be guided to an integrator sensor comprised of a photoelectric sensor, the light exposure on the wafer is indirectly detected through this integrator sensor, and emission of light is carried on before an integrated value of this detection result exceeds a predetermined level (critical level) corresponding to a light exposure necessary for the photosensitive material (hereinafter referred to as "set light exposure"). (In the case of the continuous light, the shutter is closed when the integrated value exceeds the critical level.)
When the pulsed laser light source is used as an illumination light source, there are variations in energy among pulsed laser beams. Therefore, desired repeatability of control accuracy of light exposure is achieved by irradiation with a certain fixed number (hereinafter referred to as "minimum exposure pulse number") or more of laser beam pulses. In this case, for example, when a high-sensitivity photoresist is exposed to light, the set light exposure is small; if the laser light pulse is used just as it is, the light exposure even in the minimum exposure pulse number will exceed the set light exposure, which will become an overexposure. In such cases of small set light exposure, the pulsed laser light was attenuated, for example, by a light attenuating mechanism set in the optical path, whereby appropriate exposure level was able to be made by pulses in the number equal to or higher than the minimum exposure pulse number.
In recent years, in order to transfer a larger-area pattern onto the wafer at high accuracy without increasing the scale of the projection optical system, the projection exposure apparatus using the step-and-scan method was also developed in the arrangement for successively transferring the pattern of reticle to each shot area on the wafer by synchronously scanning the reticle and wafer relative to the projection optical system in such a state that a part of the pattern of reticle is projected through the projection optical system onto the wafer. In this scanning type projection exposure apparatus, the light exposure control with focusing attention on only a point on the wafer is not adaptable, so that the aforementioned cut-off control cannot be applied. Then a first control method was a method for controlling the light exposure by simply integrating the quantity of each pulsed illumination beam (an open light exposure control method). A second control method adopted was a method for controlling the energy of each pulsed illumination beam by measuring in real time the integrated light exposure for a region included in a slit illumination field (exposure area) in the scan direction on the wafer every pulsed illumination beam and individually calculating a target energy of the next pulsed illumination beam, based on the integrated light exposure (a pulse-by-pulse light exposure control method).
In the first control method of the former, the pulse energy needs to be finely adjusted so that the following relation holds, i.e., so that the exposure pulse number is an integer, in order to achieve desired linearity of light exposure control. EQU (set light exposure)=(pulse number).times.(average energy of one pulse).times.(adjusted rate of pulse energy (modulation rate))(1)
In this equation the average energy of one pulse is a value indirectly measured by use of the aforementioned integrator sensor immediately before exposure. The output of the integrator sensor is calibrated with respect to a reference illuminance meter set on the image plane. In the second control method of the latter, the pulse energy needs to be finely adjusted every emission of pulsed beam. Therefore, either method requires an energy fine adjuster in the optical path of pulsed beam.
Each of FIGS. 7A and 7B illustrates a conventional energy fine adjuster. Among them, the fine adjuster of a double grating method shown in FIG. 7A is arranged in such a configuration that a fixed grating plate 52, in which transmitting portions and interrupting portions are alternately formed each at predetermined pitch, and a movable grating plate 51, which is movable in the pitch direction of grating, are superimposed on the optical path of laser beam LB emitted in the pulsed form and that the transmittance for the laser beam LB can be finely adjusted by shifting the two grating plates 51, 52 relative to each other The fine adjuster of FIG. 7B is arranged in such a configuration that two glass plates 53, 54, the both surfaces of each of which are coated with an antireflection coating, are placed in symmetry and both in an inclined state at a variable inclination angle .theta. with respect to the symmetry axis, on the optical path of the laser beam LB. The overall transmittance with respect to the optic axis of the laser beam LB is finely adjusted by controlling the inclination angle .theta., utilizing such a property that the transmittance of the glass plates 53, 54 varies depending upon the angle of incidence of beam. A further device for modulating the output power of the laser light source itself was also proposed as another example of the fine adjuster of pulse energy.
Further, a third control method is a method for controlling the light exposure every shot area by changing setting of the energy fine adjuster or the scanning speed of the stage, based on the light exposure measured in a shot area immediately before one of interest (a shot-by-shot light exposure control method). Since the cut-off control as in the full wafer projection exposure apparatus cannot be applied to the scanning projection exposure apparatus, the light exposure control is normally an open loop control, and all of the first to third control methods described above are techniques necessary for the open loop light exposure control.
In the earlier technology as described above, when the excimer laser light source is used as an illumination light source, the device of the excimer laser light source is larger than that of the high-pressure mercury lamp, which is one of typical illumination light sources theretofore, and the excimer laser light source necessitates gas pipe facilities or the like; therefore, in general, the excimer laser light source is set as separated from the main body of exposure apparatus and the laser beam from the laser light source is supplied to the main body of exposure apparatus while being routed through a beam sending optical system. This means that a vibration control system of the main body of exposure apparatus is separated from that of the laser light source If during successive irradiation in respective shot areas on one wafer the wafer stage moves according to positions of the shot areas, so as to cause inclination in the posture, or deformation of the main body of exposure apparatus, deviation will occur in the relative position and angle between the laser light source and the main body of exposure apparatus, thereby causing a so-called optic axis offset. The effect of this optic axis offset is not great enough to affect the performance of the imaging system of the exposure apparatus, but it could cause several %-change in an uptake amount of the laser beam into a fly's eye lens for uniforming the illuminance distribution, with movement of the irradiated position during irradiation on one wafer. Namely, the direction and amount of the optic axis offset correlate with the irradiated position on the wafer. A varying amount of the light exposure within the wafer is determined by the amount of optic axis offset, and asymmetry and an initial adjustment state of intensity distribution of the laser beam.
FIGS. 8A and 8B are explanatory drawings of the optic axis offset. In FIG. 8A, the main body of exposure apparatus is installed in body frame 55. For convenience of description, the figures show only wafer stage 56 out of the components in the main body of exposure apparatus, and fly's eye lens 57 in an illumination system in the main body of exposure apparatus, in the body frame 55. The laser beam LB emitted from excimer laser light source 58 set outside the body frame 55 is guided through the beam sending optical system including mirror 59 etc. to illuminate illumination area 60 of the entrance plane of the fly's eye lens 57 in the body frame 55. In the state shown in FIGS. 8A, the wafer stage 56 is located in the center of the body frame 55 and no optic axis offset appears. Thus, the entrance plane of the fly's eye lens 57 is within the illumination area 60 of the laser beam LB.
In contrast with it, for example, in the case wherein a position of a shot area to be exposed on the wafer is very far from the shot area exposed immediately before it, when the position of the wafer stage (precisely, a moving part thereof) 56 largely moves within the body frame 55 as shown in FIG. 8B, the body frame 55 can slightly deform (or be distorted) or be inclined by change in the mechanical center of gravity or the like. When the body frame 55 deforms in this way, deviation (the optic axis offset) occurs between the optic axis of the illumination system and the optic axis of the external excimer laser light source 58, so that the entrance plane of the fly's eye lens 57 may be off the illumination area 60 of the laser beam LB. Then, the light exposing the shot area of interest decreases in the open loop light exposure control described in the conventional example. If the position of the wafer stage 56 varies depending upon the position of the shot area to be exposed on the wafer as described above, the short-term optic axis offset will appear therewith, causing variation in the light exposure.
Further, for example, in the case wherein the laser light source is located on a floor different from that where the main body of exposure apparatus is placed, middle-term and long-term optic axis offsets will also appear due to secular change. The variation in the uptake amount into the fly's eye lens will take place in the composite form of the short-term, middle-term, and long-term optic axis offsets described above. In the case of the open loop light exposure control, such variation in the uptake light amount will result in variations in an average light exposure given to each shot area.
Even with such variation in the light exposure and, for example, when the positions of shot areas to be exposed almost continuously change on the wafer, the necessary light exposure control accuracy can be achieved by applying the third light exposure control method (the shot-by-shot light exposure control method) described previously to finely modulate the pulse energy, based on a measured-value of light exposure in the shot area immediately before. However, in the case wherein locations of plural shot areas to be exposed are discrete on one wafer and they are greatly distant from each other, for example as in the case of production of application specific IC (ASIC), variation in the average light exposure will appear due to a small optic axis offset when the wafer stage is largely moved in order to move the next shot area to the illumination field from the shot area exposed immediately before it; and there is the possibility that the correction technique on the basis of the information in the shot area immediately before could fail to accurately correct the light exposure.
In the case of successive irradiation on wafers in one lot, one wafer is changed to another and shot area exposure is normally started from the same position in the next wafer. On this occasion, even assuming that a light exposure in the first shot area of the next exposed wafer is corrected based on the exposure result in the final shot area of the previously exposed wafer by the third light exposure control method described above, there will arise a problem that the variation in the average light exposure due to the optic axis offset is not corrected well, because the irradiated position of the first shot area is greatly different from that of the final shot area in the wafer.
Further, in the case wherein the wafer stage is stepped at high speed and, for example, on the occasion of change in the column or the row of the shot area to be exposed, the optic axis offset will appear prominent, because the posture of the main body of exposure apparatus greatly changes instantly because of reaction force. The variation in the light exposure will be also large at this time, and there will arise a problem that such variation in the light exposure cannot be corrected by the shot-by-shot light exposure control method.
The variation in the light exposure described above is one caused by the optic axis offset due to the change in the posture of the main body of exposure apparatus during the exposure operation and, in addition to it, it is known that the average output of the excimer laser light source itself also varies during the exposure and also varies with a lapse of time. Accordingly, in order to perform the light exposure control at higher accuracy, the control needs to be carried out while discriminating the output variation of the illumination light source itself from the variation of actual irradiation energy on the wafer due to the other factors than the output variation.